1. Field of the Invention
The present invention relates generally to a variable capacity type refrigerant compressor suitable for being incorporated in a refrigerating circuit of an automobile climate control system, and provided with a capacity control valve for adjustably changing the compressor capacity as required. More particularly, the present invention relates to a capacity control valve for controlling the discharge capacity of a variable capacity refrigerant compressor by controlling the pressure prevailing in a compressor crank chamber in which a pressure responsive piston-reciprocating-mechanism is incorporated.
2. Description of the Related Art
An example of a conventional variable capacity refrigerant compressor suitable for being incorporated in a refrigerating circuit of a climate control system of automobiles is disclosed in U.S. Pat. No. 4,428,718 to Skinner. This variable capacity compressor is provided with a cylinder block in which a plurality of cylinder bores for receiving reciprocating pistons are arranged parallel to a central axis of the cylinder block. The axial ends, i.e., front and rear ends of the cylinder block are air-tightly closed by front and rear housings, respectively, and therefore, a crank chamber is arranged between the front housing and the front end of the cylinder block so as to receive therein a piston-reciprocating-mechanism including a drive shaft: rotatably supported by the front housing and the cylinder block. The piston-reciprocating-mechanism further includes a drive plate member mounted around the drive shaft in such a manner that it is able to rotate together with the drive shaft and to change the inclination angle thereof from a plane perpendicular to the axis of rotation of the drive shaft. The drive shaft supports thereon, via a thrust bearing, a non-rotatable wobble plate to which a plurality of reciprocating pistons slidable in the respective cylinder bores are operatively connected by respective connecting rods.
The rear housing is provided with a suction chamber and a discharge chamber formed therein which are communicated with the cylinder bores via suction ports and discharge ports formed in a valve plate arranged between the rear end of the cylinder block and the rear housing. The rear housing receives therein a capacity control valve for adjustably changing the compressor capacity, which corresponds to a typical conventional control valve as shown in FIG. 9.
The capacity control valve of FIG. 9 is provided with a suction pressure detecting means formed by bellows 91 responsive to a change in the gas pressure Ps of a refrigerant gas entering the compressor, a gas-supply passageway 92 arranged between the discharge and crank chambers of the compressor, a gas-extraction passageway 93 arranged between the crank and suction chambers of the compressor, and a valve mechanism 94 provided for controlling the closing and opening of both passageways 92 and 93, in response to a movement of the bellows 91.
The capacity control valve of FIG. 9 is also usable as a capacity control valve incorporated in a variable capacity refrigerant compressor disclosed in Japanese Unexamined utility Model Publication (Kokai) No. 62-31782 (i.e., JU-A-'782). The capacity control valve of the compressor of JU-A-'782 is substantially the sac as the control valve of FIG. 9 except that it is provided with a pressure detecting mechanism arranged so as to detect the gas pressure of the refrigerant gas at the gas outlet of an evaporator of an automobile climate control system. Therefore, a further description of the prior art control valve for a variable capacity refrigerant compressor will be given below with reference to FIG. 9.
In the above-mentioned variable capacity refrigerant compressors, when the environmental temperature is increased, and when the pressure Ps of the refrigerant gas detected at either the refrigerant gas inlet of the compressor or the gas outlet of the evaporator is higher than a predetermined pressure level, the bellows 91 of the control valve of FIG. 9 is compressed so as to move the valve mechanism 94 to open the gas-extraction passageway 93 and to close the gas-supply passageway 92. Thus, the refrigerant gas at a rather high pressure moves from the crank chamber toward the suction chamber to thereby reduce the pressure level prevailing in the crank chamber. As a result, when the pressure on the backs of the respective reciprocating pistons is reduced, the reciprocating strokes of the respective pistons are increased by increasing the inclination angle of the non-rotatable wobble plate supported on the drive plate member. Thus, the compressor capacity is increased.
On the other hand, when the pressure Ps of the refrigerant gas detected at either the refrigerant gas inlet of the compressor or the gas outlet of an evaporator of the refrigerating circuit is reduced to a predetermined pressure level due to reduction in the environmental temperature, the bellows 91 (the suction pressure responsive element) of capacity control valve expands to move the valve mechanism 94 to a position where the gas-extraction passageway 93 is closed, and the gas-supply passageway 92 is opened. Thus, the compressed gas having a discharge pressure Pd in the discharge chamber of the compressor is introduced into the crank chamber of the compressor so as to increase a gas pressure Pc prevailing in the crank chamber, and accordingly, the backs of respective pistons are subjected to a higher gas pressure Pc. Thus, the reduced piston strokes of respective pistons combines to reduce the inclination angle of the non-rotatable wobble plate. Therefore, the compressor capacity is reduced.
Therefore, in the variable capacity type refrigerant compressor provided with the above-mentioned conventional capacity control valve as shown in FIG. 9, the relationship between the environmental temperature and the suction pressure Ps of the refrigerant gas, detected at either the gus inlet of the compressor or at the gas outlet of the evaporator, is established as shown in a graph of FIG. 11. In FIG. 11, Psc indicates the suction pressure of the refrigerant gas detected at the inlet of the compressor, and Pse indicates the pressure of the refrigerant gas detected at the outlet of the evaporator.
At this stage, with the control valve for a variable capacity refrigerant compressor as shown in FIG. 9, when the effective pressure receiving area of the bellows 91 is S1, a spring force set within the bellows 91 is F1, the cross-sectional area of the gas-extraction passageway 93 of the valve mechanism is S2, the cross-sectional area of the gas-supply passageway 92 of the valve mechanism 94 is S3, a spring force applied to the lower end of the valve mechanism 94 is F2, the suction pressure of the refrigerant gas is Ps, the pressure of the refrigerant gas in the crank chamber is Pc, and the discharge pressure of the refrigerant gas is Pd, the equation (1) can be defined as set forth below. EQU F1=S1Ps+S2(Pc-Ps)+S3(Pd-Pc)+F2 (1)
Therefore, the following equation (2) can be derived from the above equation (1). EQU F1=(S1-S3)Ps+S3Pd+(S2-S3) (Pc-Ps)+F2 (2)
Thus, from the equation (2), it is understood that the spring force F1 varies with a change in the discharge pressure Pd. Further, it is understood from the construction of the control valve of FIG. 9 that the opening area Of the gas supply passageway 92 is determined depending on a force defined as S3 Pd and acting so as to close the gas-supply passageway 92.
Therefore, when the environmental temperature is relatively high, i.e., the environmental temperature is in the region A shown in FIG. 11, the discharge pressure Pd is accordingly increased, and the opening area of the gas-supply passageway 92 is determined by the increased discharge pressure Pd of the refrigerant gas. Accordingly, a large pressure drop .DELTA.P appears so as to maintain the pressure Psc at the inlet of the compressor lower than the pressure Pse at the gas-outlet of the evaporator, and the pressure Pse at the gas-outlet of the evaporator can be low and substantially constant. Therefore, the evaporator can exhibit a sufficient cooling function.
On the other hand, when the environmental temperature is low, i.e., the environmental temperature is in the region B of FIG. 11, the discharge pressure Pd is greatly decreased so as to sufficiently open the gas-supply passageway 92, and therefore, the compressor capacity is excessively reduced. Thus, the pressure Psc at the suction inlet of the compressor and the pressure Pse at the gas-outlet of the evaporator are increased so as to increase the surface temperature of the evaporator. Thus, the evaporator cannot exhibit sufficient cooling function
It should be understood that the relationship between the pressure Ps and the environmental temperature as shown in FIG. 11 is established under a condition such that the environmental temperature means the temperature of an outer air of an automobile, and the region "F" of FIG. 11 in which frost is attached to an outer surface of the evaporator should be avoided. Thus, if an automobile with an air switching unit for changing an air-flow in an automobile from an outer air-flow to an air-recirculation and vice versa depending on traffic condition, and when the air-flow is changed to the inner air-flow, i.e., the air-recirculation, the relationship between the environmental temperature and the suction pressure must be overlapped with region "C" in FIG. 11 in which an automobile's window is fogged. Namely, when the air outside an automobile is rather low, arid the air-flow is switched to the air-recirculation, for example, when a climate control system is automatically operated during winter season, in order to rapidly heat the interior of an automobile by the air-recirculation and to simultaneously dehumidify the ale in the cabin, since the outer air temperature is in the region "B" in FIG. 11, the temperature of the outer surface of the evaporator is increased, and accordingly, the air in the automobile cannot be sufficiently cooled. Thus, the inner air cannot be appropriately dehumidified,and the automobile window is fogged.
In order to solve the above mentioned problem, it may be possible to adjustably set the absolute inclination of the curves of the relationship between the suction pressure Ps and the environmental temperature at a small value to thereby prevent an increase in the pressures Psc and Pse.
Namely, from the above-mentioned equation (1), the equation (3) as set forth below is obtained. EQU Ps=-S3 Pd/(S1-S2)-(S2-S3 )Pc/(S1-S2) +(F1-F2)/(S1-S2) (3)
Thus, with respect to the equation (3), if the coordinate system (Pd, Ps) is taken as similar to FIG. 11, it is understood that an inclination of lines of the relationship between Pd and Ps will be determined by -S3/(S1-S2). Therefore, if the inclination of the lines should be set small, it is required that either S1 is set large or S3 is set small. Nevertheless, if S1 is set large, the size of the bellows 91 (FIG. 9) must be large making the entire size of the control valve large. Accordingly, such a problem occurs that it is very difficult to mount the control valve in the rear housing of the variable capacity refrigerant compressor. Moreover, the performance of the control valve, i.e., the pressure response of the control valve is apt to be degraded.
On the other hand, if S3 is made small, the cross-sectional area of the gas supply passageway 92 (FIG. 9) must be reduced to thereby reduce the flow amount of the compressed gas from the discharge chamber to the crank chamber of the compressor. As a result, the performance of the control valve must be degraded.
Further, if the inclination of the lines of the relationship between Pd and Ps is set small, the pressures pse and Psc are increased in the high-temperature region of the environmental air corresponding to the region A of FIG. 11. As a result, the refrigerating performance of the climate control system provided with a variable capacity type refrigerant is insufficient in the region A. It should here be understood that the relationship between the suction pressure "Ps" and the environmental temperature as shown in FIG. 11 is established such that the environmental temperature means the temperature of air outside the automobile, and that the region "F" of FIG. 11, where frost forms on the outer surface of the evaporator, should be avoided.
Thus, if an automobile is provided with an air change-over unit for changing the air-circulation in an automobile from external air-circulation to the air-recirculation and vice versa, depending on driving conditions, and when the air-circulation is changed to the air-recirculation, the relationship between the environmental temperature arid the suction pressure of the compressor becomes inferior.